The fuel-cell stacks used to supply electrical energy on board motor vehicles are generally of the solid-electrolyte type, especially with a polymeric electrolyte. Such a cell stack uses especially hydrogen (H2) and oxygen (O2) as the fuel and oxygen carrier respectively.
With this type of cell stack, it is possible to achieve, at the same time, an efficiency, a reaction time and an operating temperature that on the whole are satisfactory for delivering electricity to an electric motor for propulsion of a motor vehicle.
In contrast to combustion engines, which discharge a non-negligible quantity of polluting substances with the exhaust gases, the fuel-cell stack offers in particular the advantage of discharging only water, which is produced by the reduction reaction at the cathode. In addition, the oxygen carrier of a cell stack of the type described in the foregoing can be ambient air, the oxygen (O2) of which becomes reduced.
The cathode generally has an inlet that permits continuous supply with oxygen (O2) or with air, and an outlet that permits evacuation of the excess air or oxygen (O2) as well as evacuation of the water produced during the reduction of oxygen (O2). In general, the anode is generally provided with an inlet through which hydrogen (H2) is introduced.
In the current state of the art, however, the storage of pure hydrogen (H2) on board the vehicle necessitates a volume that is too large to achieve comfortable autonomy. In addition, the logistics of distribution of hydrogen (H2) have not yet become geographically widespread.
It is known that these problems can be overcome by producing hydrogen (H2) directly on board the vehicle from hydrocarbons, especially conventional fuels such as gasoline or natural gas. The hydrogen (H2) is extracted from the gasoline during an operation known as reforming, which necessitates a device known as a reformer.
The gasoline is injected into the reformer together with water and air. The product of reforming is a gas known as reformate, which is composed mainly of hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), oxygen (O2) and nitrogen (N2). The anode of the cell stack is then supplied directly with reformate by the reformer.
To be able to produce such a reformate, the reformer must be heated and maintained at a temperature higher than a threshold temperature. For this purpose, the reformer is provided with a heating device. Below this threshold temperature, the reformer cannot supply the cell stack with hydrogen (H2) fuel, and the cell stack therefore cannot produce electrical energy.
However, the threshold temperature of the reformer is higher than the ambient temperature to which the vehicle is likely to be exposed. Thus, when the reformer is cold, the heating device needs a non-negligible time, which can be as much as several minutes, to bring it to temperature. During this time, the electric motor cannot be supplied by the fuel-cell stack, and the operator must wait until the reformer is operating before he can use the motor vehicle.
To permit the operator to use the vehicle quickly after it has been started, it is known that the vehicle can be equipped with a battery of auxiliary accumulators in order to supply the electric motor during the time for heating the reformer. Thus, during heating of the reformer, the motor is supplied with electrical energy by the auxiliary battery and, when the reformer has reached its threshold temperature, the electrical-energy supply source automatically switches from the battery to the fuel-cell stack.
However, such a battery generally cannot deliver as much electrical power to the motor as a fuel-cell stack. This has consequences for the maximum power that the motor can deliver instantaneously and thus for the driving sensations of the operator.
To permit the operator to control the electric motor, the vehicle is generally provided with an accelerator pedal, which can be moved between a rest position and a maximum position, which corresponds to the maximum power that can be delivered by the motor as a function of the electrical power that can be released by the fuel-cell stack. The pedal is also capable of occupying a threshold position, which is situated between the rest position and the actuated maximum position, and which corresponds to the maximum power that can be delivered by the motor when it is being supplied by the battery.
When the motor is being supplied by the battery, the actuation of the accelerator pedal from the rest position to the threshold position is felt as a continuous increase of power by the operator. The actuation of the accelerator pedal beyond the threshold position then has no effect on the power delivered by the motor, contrary to what the operator expects when the motor is being supplied normally by the fuel-cell stack.
In addition, when the supply source of the motor changes over automatically from the auxiliary battery to the fuel-cell stack, and when the accelerator pedal is actuated beyond the threshold position, the motor is suddenly supplied by a greater electrical power. The abrupt increase of power delivered by the motor as a consequence is then capable of surprising the operator and/or of causing an accident.
U.S. Pat. No. 6,447,939 B1 describes a device in which, when a “quick down” is detected during startup of the reforming phase, the quantity of electrical energy necessary for startup of the reformer is limited and the quantity of electrical energy distributed to the motor is increased by giving priority to the supply of the motor.